This application claims the priority of German patent document 199 22 959.7, filed May 19, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an exhaust-gas cleaning system. Exhaust-gas cleaning systems are used, for example, for internal-combustion engines of motor vehicles and enable any nitrogen oxides contained in the exhaust gas to be cleaned out using the so-called SCR (Selective Catalytic Reduction) process. In this process, the nitrogen oxides are chemically reduced in a suitable reduction catalytic converter with the participation of a suitable reducing agent which is added to the exhaust-gas flow. Ammonia in vapour or gas form is often used as the reducing agent and is generated by the vaporization and subsequent thermolysis and hydrolysis of urea which has preferably been introduced into the exhaust-gas flow in solution.
An exhaust-gas cleaning system of this type is disclosed in publication WO 97/36676. In this system, liquid urea solution is vaporized in an electrically heatable metering unit, which is arranged on an exhaust pipe wall, with the urea being hydrolysed and is injected into the exhaust-gas flow. A downstream mixer assists with mixing the injected reducing agent with the exhaust gas.
EP 0,555,746 B1 discloses a metal honeycomb vaporizer, which is heated by the exhaust gas and serves simultaneously as a hydrolysis catalytic converter for injected urea solution and a flow mixer. Downstream of the metal honeycomb vaporizer body in the exhaust system there is a multipart catalytic converter unit which comprises an upstream nitrogen oxide reduction catalytic converter and a downstream oxidation catalytic converter.
In an exhaust-gas cleaning system which is disclosed in Patent EP 0,615,777 B1, urea is injected into the exhaust-gas flow in the form of solid particles, which is then fed to a hydrolysis catalytic converter and, from there, to a nitrogen oxide reduction catalytic converter and a downstream oxidation catalytic converter.
The invention is based on the technical problem of providing an exhaust-gas cleaning system in which the reducing agent can be added to the exhaust-gas flow as a vapour in an advantageous manner and with relatively little outlay for the purpose of nitrogen oxide reduction.
The exhaust-gas cleaning system of the present invention solves this problem. In the system according to the present invention, the vaporizer which is arranged in the exhaust system upstream of the nitrogen oxide reduction catalytic converter contains, as its heat source, either a heatable deflector surface, onto which the reducing agent, which is supplied by a feed unit, is directed under pressure, or a microwave generator. It has been found that, firstly it is possible to achieve sufficient vaporization of the reducing agent and, secondly, the cost of providing the vaporizer remains relatively low.
In an embodiment of the exhaust-gas cleaning system, the heat source provided for the vaporizer is a baffle plate which is oriented parallel to the direction of flow of the exhaust gas and therefore does not significantly disrupt the exhaust-gas flow, so that there is no significant drop in the exhaust-gas pressure across the vaporizer. The baffle plate may be positioned with surface-to-surface contact against the inside of an exhaust pipe or may be positioned in the interior of the latter. The heating may be effected in a conventional way which is suitable for the particular application, for example by means of an associated electrical heater unit.
In another embodiment of the exhaust-gas cleaning system, the heat source provided for the vaporizer is a heatable baffle plate which is situated in a part-flow branch line of the exhaust system, this branch line branching off from a main-flow exhaust system section and opening back into this section downstream of the branching point.
Another embodiment of the exhaust-gas cleaning system contains at least two catalytic converter stages which are connected one behind the other, having different ammonia storage performances and at least one of which forms the nitrogen oxide reduction catalytic converter. It has been found that in certain cases this measure is advantageous in order to achieve the maximum possible reduction of nitrogen oxide, and, depending on the particular application, the upstream stage may have a higher or lower ammonia storage capacity than the downstream stage.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.